As seen in FIG. 12, a conventional passive mode pyroelectric temperature sensing system (e.g. a night vision system) employs a pyroelectric sensor 120 that operates either cryogenically to overcome noise, or at nominal room temperature for economy and convenience. The pyroelectric sensor 120 comprises a chopper 122, an infrared (IR) absorber 124, a pyroelectric element 126 (i.e. a ferroelectric transducer that exhibits a symmetrical hysteresis loop), a current integrating capacitor 128, and a baseband amplifier 129. The pyroelectric element 126 is typically implemented in a Sawyer-Tower circuit design and is homogenous in nature.
The input of the pyroelectric sensor 120 is an IR radiation of temperature. The output of the pyroelectric element 126 is a quantity of charge as a function of time, Q(t), which is shown as an input for the baseband amplifier 129. The output of the baseband amplifier 129 is an electrical analogue of incident IR.
The pyroelectric sensor 120 operates as a static DC device, where a change in radiated infrared temperature causes a slight change in static bound-charge. As shown in FIG. 13, the differential area, A, of a saturated pyroelectric hysteresis loop, represents the polarization energy stored in the sensor 120. The differential area, A, of the symmetrical hysteresis loop is a direct function of radiated infrared temperature, T1, T2 (FIG. 14).
Referring to FIG. 14, if T2 is greater than TI, the hysteresis loop will exhibit a size reduction and decrease in polarization, which is the charge, Q. In such a conventional temperature sensing system, a change in radiated infrared temperature T1, T2 causes a slight change in static DC bound-charge Q1, Q2. However, a mobile free charge, which also exists in the system, flows at some definite time constant and tends to neutralize the change in bound-charge. Therefore, to deceive the human eye, the pyroelectric sensor 120 is reset in temperature at a frequency (approximately 15 Hz) in order to detect a change in radiated infrared temperature T1, T2. The change in radiated infrared temperature T1, T2 is detectable provided that the chopper 122 overcomes the time-constant of the sensor 120. Consequently, the very low frequency of the temperature-reset, dictates a sluggish, inefficient response.
As shown in FIG. 15, the area of the symmetrical hysteresis loop is represented for a case where the sensor 120 is driven from plus saturation, Ps, to minus saturation, −Ps. In such a case, it was discovered that with special circuitry (i.e. a rectifier, charge amplifier, and charge integrator), an active mode AC operation of homogeneous ferroelectric transducer resulted in a huge pseudo-pyroelectric effect. Instead of operating statically at some point on the hysteresis loop and noting the change in charge (i.e. Q1, Q2) with temperature (i.e. T1, T2), the entire loop was driven at a high frequency and the change in the loop area with temperature was noted. Because the sensor 120 is operated with alternating excitation, the accumulated area of the hysteresis loops per unit time can be immense. Temperature resetting is not required in such a case, thus increasing the signal to noise ratio.
For active mode AC operation of the homogeneous ferroelectric transducer, the electric field is cycled instead of temperature. Typically, the temperature of the scene is continuous but dithered; however, the value being sought is temperature. In such a case, while the temperature is dithered, the electric field is cycled. To achieve the latter behavior from a homogenous ferroelectric transducer, the special circuitry is typically employed in order to sum the areas of a hysteresis loop, wherein the areas of the loops are defined by a monotonic function of temperature.
It was also noted that by taking advantage of the AC traversing of the hysteresis loop many times per second, the signal to noise ratio for temperature sensing is immensely improved. More importantly though, as the speed of the response is greatly increased, in theory, specific applications can be achieved (e.g. rocket flight can be detectable in a night vision application).
Further, the area of the hysteresis loop of FIG. 15 changes most rapidly with temperature as the sensor 120 approaches the Curie temperature, or more specifically, the temperature that causes a maximum change in polarization per change in temperature. Graphically, the Curie temperature is approximately located at the point of maximum slope of the shrinking hysteresis loop near the temperature axis. The temperature range where the loops are most responsive to a change in temperature is approximately at or near the Curie temperature. Thus, the Curie temperature heavily factors into determining the sensitivity of the sensor.
In the example discussed above for FIGS. 12–15, the area of the hysteresis loop for a homogeneous ferroelectric transducer may result in a huge pseudo-pyroelectric effect and is most responsive to a temperature range that is at or near the Curie temperature. Although an active mode AC operation of homogeneous ferroelectric transducers may result from the use of special circuitry, it may not be easy to acquire the temperature for the most sensitive hysteresis loop at or near the Curie temperature. For a given ferroelectric transducer, the Curie temperature is fixed, while the quiescent temperature of the sensor (i.e. operating temperature) may vary considerably.
Even further as seen in FIG. 16, while the hysteresis loop is in saturation, a conventional pyroelectric sensor 160 that is driven with an AC excitation from a source 162 may not consistently convert received infrared energy to bound-charge. For efficiency, the sensor 120 must be converting radiated or conducted heat to bound-charge 100% of the time. Consequently, the hysteresis loop is driven just short of saturation, which effects the sensitivity of the pyroelectric sensor 160. For further reference, pyroelectric sensors are described in detail in U.S. Pat. Nos. 6,294,784 and 6,339,221 to Schubring et al.
Therefore, it is the objective of the applicants to overcome the fallbacks of conventional homogenous pyroelectric sensing systems by maximizing the sensitivity of the sensor during active operation. Even further, it is also contemplated by the applicants that the fallbacks of conventional homogenous ferroelectric transducer sensing systems may be overcome by implementing graded ferroelectric transducers in the Sawyer-Tower circuit configuration.